This invention pertains to an effective method to electrochemically synthesize quinone compounds from aromatic compounds using a paired electro-oxidation approach in an undivided cell.
The two primary techniques for oxidizing an aromatic compound to a quinone compound are: (1) the catalytical oxidation method; and (2) the electro-oxidation method. The first method requires high temperature, high pressure, and catalysts. Maintaining this high temperature and pressure results in high cost, difficulty of operation, and poor selectivity of resultant products.
Using an electrochemical method to convert an aromatic compound to a quinone compound has several advantages including low temperature and pressure, easy to operate, and high selectivity of resultant products.
The electro-oxidation approach is to oxidize an aromatic solid or its organic solution in an anode of an electrochemical cell to produce a quinone. The electrolyte solution is composed of a redox mediator solution and an aromatic compound (solid or solution). The redox mediator solution can be an aqueous inorganic redox couple in solution such as Mn.sup.3+ /Mn.sup.2+, Ag.sup.2+ /Ag.sup.+, Ce.sup.4+ /Ce.sup.3+ V.sup.5+ /V.sup.4+, Co.sup.3+ /Co.sup.2+, and Tl.sup.3+ /Tl.sup.2+. The low valence state metal ions of the redox mediator are oxidized first on the anode surface and become higher valence ions. Then the oxidized ions react with the aromatic compounds and form quinone and lower valence ions. The resultant product, i.e., the quinone compound, can be removed from the solution, and simultaneously the lower valence ions can be recycled into the cell for another oxidation cycle. The cycle can proceed continuously until the aromatic compound is completely exhausted.
Among the chemical reactions, quinone is formed on the anode surface, and the cathodic reaction involves a fine metal wire (as cathode) and the reduction reaction of water, which forms hydrogen; or alternatively, to reduce some undesired compound so that the reaction on the anode surface would not be affected. Consequently, the process becomes inefficient, resulting in a poor distribution of the system voltage and current, and the waste of electric energy on the cathode.
U.S. Pat. No. 4,950,368 provided a paired electro-oxidation method to synthesize ethylene glycol on the cathode from the reduction of formaldehyde in an electrochemical cell that had a dialysis membrane installed in the middle. Simultaneously, at the surface of the anode, higher valence metal ions are formed. These ions can oxidize another compound and produce a second product. This paired electro-oxidation approach can increase efficiency and save energy; however, there are still disadvantages. For example, the resultant products of both electrodes are not the same, requiring further separation.
In the R.O.C. Patent No. 30226 (application No. 76100506), equivalent to J. of Applied Electrochemistry 17 (1987) 753-759, the detailed procedure of a paired electro-oxidation synthesis method for the production of aromatic aldehyde is described. The method, which generated a Fenton's reagent, could be used to oxidize toluene to benzaldehyde. Simultaneously, at the anode the Mn.sup.2+ was oxidized to Mn.sup.3+, and Mn.sup.3+ could continuously oxidize toluene to benzaldehyde. The Mn.sup.2+ ions were reoxidized at the anode again and could be used repeatedly.
The purpose of this invention is to develop a paired electro-oxidation method to synthesize a quinone compound effectively from an aromatic compound. An improved reactor design for the paired electro-oxidation reactions is employed, i.e., reactions take place in an undivided single cell without the presence of any membrane.